Reports the results of an analysis of combined mortality data on 95,673 nuclear industry workers at seven facilities in the USA, UK and Canada. Workers at these facilities, which include both atomic energy stations and nuclear weapons plants, have been monitored in previous studies to determine the health effects of external exposure to ionising radiation. By combining and assessing data from previous studies, the present analysis aimed to develop more precise estimates of the risk of radiation-induced cancer and thus to strengthen the scientific basis for setting radiation protection standards. The study also compared risk estimates among workers with estimates obtained in high-dose studies, including investigations of atomic bomb survivors and of patients irradiated for therapeutic purposes. The results represent the most comprehensive and precise direct assessment to date of the carcinogenic risk of protracted external exposure to generally low doses of ionising radiation. The combined analysis of data on nuclear industry workers found a significant increase in the risk of leukaemia, and of myeloid leukaemia in particular, at relatively low dose levels. The study also provided an opportunity to examine some of the previously reported associations between low doses of ionising radiation and mortality from specific cancer types. Of the 36 cancer types or groupings considered, most showed little or no association with radiation exposure. Apart from leukaemia, multiple myeloma was the only cancer to exhibit a statistically significant association with radiation dose.